Advantages of optical sensors in harsh environments containing high voltages, high electromagnetic interference (EMI), corrosive materials and other degrading components are well known and produce desirable benefits. Several types of optical force sensors and angle sensors exist in the prior art. As one example, a Fabre-Perot (FP) etalon-type sensor can be used for the measurement of pressure. Fabre-Perot sensors are generally phase-sensitive devices. They are often interrogated by specialized spectrometers to prevent their signals from being intensity dependent (and thus vulnerable to intensity noise and attenuation) or limited to the narrow range of one interference fringe. Fabre-Perot fringes can also be counted to obtain information as the measurand changes. If the count is lost, such as during a power failure, the sensors can be re-initiated under known conditions. Multiplexing of FP sensors or sensors can be challenging, usually employing an optical fiber for each sensor selected by an optical switch or an optical splitter. This can sometimes cause increased complexity, for example in feeding the optical signals from multiple sensors through bulkheads with multiple fibers. The construction of such sensors also often relies on precise control of the very small gap between the mirrors.
Other fiber optic pressure sensors use pressure on a diaphragm to influence a body that blocks part of the light in the light path. Such intensity-based optical sensors, including other intrinsic and extrinsic fiber optic sensors, can be subject to interference from other sources of intensity loss, such as in fiber bending, dirt, variable connectorization losses, and optical fouling.
Other methods use fiber Bragg gratings (FBGs) in a pressure sensor. FBGs are strain sensitive, leading to their use as strain and other force-actuated sensors. While many FBGs can be multiplexed easily on a single fiber by several means, when an optical fiber needs to be stretched or compressed it can sometimes be difficult to fix the fiber portions near the FBG in a compact way and without breakage or without the fiber slipping (creeping) though the fixing material under stress. It is well known that such slippage occurs with epoxies and other adhesives, and even metal solders. Attempts to fix and hold the bare fiber with melted glass may result in breakage. FBGs are also temperature sensitive, necessitating the use of temperature compensation. The most sensitivity to strain for FBGs is often in the axial direction. For tensile strain, the measurement is relatively simple to perform if the fiber can be gripped firmly without slippage. For compressive strain, the fiber will be pre-stressed in order to prevent fiber buckling. For long periods of time, this can lead to vulnerability to breakage. In addition, a sensor that employs the stretching of an FBG to measure pressure frequently requires the mechanical translation of a compressive or tensile force in the direction of the fiber diameter into an expansion movement along the fiber axis, which can be difficult to accomplish. FBGs can also be utilized in a bending mode, but creep and wear are difficulties with this method as well.
It is advantageous in exemplary illustrative non-limiting force sensor implementations to include a wavelength dependent structure (WDS) that can either reflect, transmit, diffract or refract all or part of an optical spectrum incident on the WDS and can be dependent on the orientation and/or location of the WDS with respect to an interrogating light beam.
The nature by which the WDS reflects, transmits, diffracts or refracts the spectrum of the interrogating light beam can be one that is non-exclusively wavelength resonant, dispersive or dichroic in any given orientation with respect to said light beam. A WDS based on optical resonance can be termed a “resonant wavelength optical device” structure, which may include optical gratings, planar interference filters or wedged interference filters. More specifically, other WDS can incorporate, but are not limited to, dispersion-based optical structures, such as prisms, dichroic-based structures such as multilayer optical filters or mirrors, and diffraction-based optical structures, such as ruled, stamped, Rugate, Echelle, Littrow, holographic, volume-phase or the like gratings.
Exemplary illustrative non-limiting optical sensor implementations incorporate at least one wavelength-sensitive element that may be constructed on a substrate and further may be attached to at least one more substrate for various specific purposes, creating a wavelength dependent structure (WDS) that reflects or transmits a wavelength-specific spectral feature (as a non-exclusive example, a resonant peak or valley). Further, the orientation of said WDS can be measured in reference to an interrogating light beam direction by means of said wavelength-specific spectral feature. Said orientation can be induced by a force on said WDS in a pre-arranged manner, thereby transducing said force to a wavelength-encoded optical signal. Said wavelength-encoded optical signal may be further translated to an electrical signal by an optical detector and then to a quantitative representation of said force by further electronic circuitry and/or software.
In example non-limiting implementations, said reference direction can be defined by a beam of interrogating light through free space, a lens system, a mirror system, an optical fiber, or a like optical path arrangement. The light reflected or transmitted as a wavelength-specific spectral feature can be returned to an instrument that measures the reflected or transmitted light spectrum and translates the optical spectral feature into the orientation of the WDS by either the same path as the incident interrogating light or by a different path. The instrument can be additionally configured or calibrated to translate or transduce the wavelength-specific spectral feature of the measured wavelength dependent structure into such force that may cause any changes in the orientation of said wavelength dependent structure. Such forces can non-exclusively include pressure, vibration, fluid flow, magnetic field, electric field, direct mechanical force, displacement, inclination (tilt), acceleration, weight, strain and/or load force. A sensor apparatus may employ as non-exclusive examples levers, pivots, bearings, flexure hinges and linkages, diaphragms, bellows, fluid transmission of said force, magnetostrictive, electrostrictive or other known means to influence the orientation of said wavelength dependent structure. Said sensor apparatus may incorporate multiple measurements and may include temperature compensation by either mechanical design or an independent temperature sensor.
The attributes of subject novel non-limiting sensors include adaptability, wavelength encoded signals, separation of the force-sensitive element from the fiber to avoid strain-temperature cross sensitivity (as with FBGs), and retention of optical sensors' well-known insensitivity to high voltages, electromagnetic interference (EMI), corrosion and other benefits.
One example non-limiting implementation provides an optical sensor for use with a light source providing an interrogating light beam that illuminates a wavelength dependent structure which in turn produces a wavelength-specific spectral feature in conjunction with said light source, said sensor comprising a wavelength dependent structure attached to or incorporated into a mechanism the orientation of which can be made to change in relation to the interrogating light beam by the application of a force, the wavelength dependent structure possessing an optical property that changes the spectral content of transmitted, reflected or refracted light with respect to orientation; at least one optical detector; and at least one return optical path that carries said at least one wavelength-specific spectral feature to the at least one optical detector, said wavelength dependent structure in use being moved in orientation with respect to the light beam by an applied force, thereby causing a shift of spectral content of said wavelength-specific spectral feature, wherein said shift in spectral content is utilized to translate said applied force to a physically meaningful parameter by means of sensor mechanical design, the interrogating light beam, an optical detector and additional electronic circuitry and software.
Said incident light beam may possess a broad spectral band and said optical detector may comprise a wavelength-sensitive optical detector.
The light source may comprise a swept wavelength laser and said optical detector may comprise a simple photodiode detector.
Said interrogating light beam in conjunction with said optical detector and WDS may be structured to use said shift in spectral content to translate said applied force to a physically meaningful parameter.
A non-limiting example sensor of an exemplary illustrative non-limiting implementation may further include means for providing temperature compensation. Said temperature compensation means may include at least one of a mechanical compensation means and an optical temperature sensor mounted in a strain-free manner independent of the force to be measured. Said optical temperature sensor may non-exclusively include one or more of a fiber Bragg grating and a Fabre-Perot sensor; a semiconductor bandgap optical temperature sensor, a fluorescent time decay temperature sensor or a WDS.
The sensor may include a calibrating component that calibrates the spectral changes of the wavelength-dependent device to the applied force.
The wavelength-sensitive element component of a wavelength dependent structure (WDS) may non-exclusively be selected from the group consisting of a ruled grating, a Littrow grating, a volume phase grating, a holographic grating, a Rugate filter or mirror, a photonic crystal, a planar Bragg mirror, a Bragg mirror incorporating multiple mirrors with phase shift cavities between them, a Bragg transmission filter, a linear variable filter, a non-linear variable filter, and combinations thereof.
A detector may be comprised, as non-limiting examples, of at least one of a spectrometer, a wavelength sensitive detector or a wavelength sensitive interferometer; a position-sensitive detector, and the detector may further include as a non-limiting example a linear variable filter coupled to any of a position-sensitive detector; a double photodiode or array of more than two photodiodes; a charge-coupled device; or a complementary metal-oxide-semiconductor device.
A light source may be selected from the group consisting of at least one of a broad spectrum non-coherent source, a light emitting diode, an amplified fluorescent stimulated source, an amplified semiconductor simulated source or a coherent laser beam from a variable wavelength laser.
A wavelength dependent structure may be arranged to change its orientation in response to an applied magnetic field.
A non-limiting example magnetic field sensor may further include at least one magnetic field-sensitive component comprised of at least one magnetostrictive element coupled to a wavelength dependent structure and in addition may include at least one electric current conductor for the purpose of providing an electric current sensor.
The wavelength dependent structure may be structured to change its orientation in response to an applied electric field.
A non-limiting example electric field sensor may be comprised of at least one piezoelectric element coupled to a wavelength dependent structure. Said piezoelectric element further can be composed of a single material, single layers, cascaded layers of the same material or layers of different materials, and force is provided by an electric field acting on said piezoelectric element to cause physical expansion or contraction and thus movement of the wavelength dependent structure.
A non-limiting example force sensor may be comprised of at least one force-transmitting component coupled to a wavelength dependent structure and further may be responsive to at least one or more types of force, including but not limited to mechanical load, gravitational force, change of momentum, fluid pressure, vibration, torque, temperature-induced expansion, acceleration, stress, or centrifugal force.
A non-limiting example rotational speed and position sensor may be comprised of at least one WDS coupled to a rotating element.
An interrogating light beam amplitude may be modulated in time.
The sensor may further include means for calibrating the spectral changes of the wavelength-dependent structure to the applied force.
A non-limiting example sensor may further include a WDS comprised of at least one wavelength sensitive element possessing a linearly graded optical structure selected from the group: ruled grating, Littrow grating, volume phase grating, holographic grating, photonic crystal, planar Bragg mirror, Bragg mirror incorporating multiple mirrors with phase shift cavities between them, a Bragg transmission filter.
A sensing system employing at least one sensor may further comprise at least one linear variable filter combined with a position-sensitive light detector to provide a wavelength-sensitive detector as a component of the interrogating instrument.
A sensing system employing at least one sensor may further comprise a photodetector in the interrogating instrument that may be selected from at least one of a single photodiode, a bi-cell photodiode, a quad photodiode, an avalanche photodiode, a photoresistor, an array of photodiodes, a charge-coupled device, or a complementary metal-oxide-semiconductor device.
A sensing system employing at least one sensor may comprise at least one light source in the interrogating instrument that may be selected from the group consisting of at least one of a broad spectrum non-coherent source, an amplified stimulated source (fluorescent or semiconductor) and a coherent laser beam from a variable wavelength-laser (tunable or swept wavelength laser).
Other non-limiting example features and advantages include:                ruggedness, compactness, competitive performance, reduced complexity and ease of adaptation to measure a variety of stimuli, while providing a signal that can be optically multiplexed        provide a wavelength-encoded optical signal that is “absolute” in the sense that wavelength changes can be measured very accurately and the signal is not vulnerable to light intensity variations as long as enough light power is preserved to actuate the detector        are wavelength-encoded and thus can be wavelength division multiplexed,        decouple the wavelength-sensitive component from the optical fiber to prevent stress-induced uncertainty and dynamic range limitations (as is common with fiber Bragg gratings, known as FBGs), and        can further be interrogated by a variety of wavelength sensitive means, including, as non-limiting examples, swept wavelength lasers combined with simple photodetectors and/or broadband light sources combined with wavelength-sensitive detectors.        